Increasing FD_SETSIZE

Marco Stornelli
Fri Oct 5 16:01:00 GMT 2012

Il 30/09/2012 10:32, Marco Stornelli ha scritto:
> Hi all,
> I need to increase the FD_SETSIZE value from 1024 to 4096. I know it'd
> be better to use poll()/epoll() but I want to understand what are
> pros/cons. The main question is: have I to recompile glibc? I read
> several thread where the change of .h after changing FD_SETSIZE works
> recompiling only the user application. Reading the glibc code (and the
> kernel too), actually it seems to me that if I want to use select(),
> FD_* macro and so on, I have to recompile all because the size of fd_set
> is changed. At this point I have to recompile *all* not only my
> application because if in the system there is an another "common"
> application that uses select and friends, I could have problem. Am I right?
> Thanks,
> Marco

Hi all,

after a search I understood the trick. FD_SIZE, FD_ZERO and so on are 
macro and not function (your application won't call an external 
library), so if you change __FD_SETSIZE you are changing the size of 
fd_set and when you compile your application everything it's ok. Glibc 
uses poll when needed so no problem. From kernel point of view there is 
no problem because it uses the first paramenter of select and then the 
max fds per-process, there isn't a problem to handle a bigger struct 
fd_set. So at the end, in your development environment you need to 
change .h and in your target environment you need to do uname -n 4096. A 
problem can happen I think, if in this condition, you try to run your 
application on a system with uname -n 1024.



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